Arachidonic acid (all-cis-5,8,11,14-eicosatetraenoic acid) is a polyunsaturated fatty acid (PUFA) containing 20 carbon atoms with four double bonds. The double bonds are arranged with the last one located six carbon atoms from the methyl end of the chain. Therefore, arachidonic acid is referred to as an omega-6 fatty acid. Arachidonic acid is one of the most abundant C.sub.20 PUFA's in the human body. It is particularly prevalent in organ, muscle and blood tissues. Arachidonic acid is a direct precursor for a number of circulating eicosenoids, such as prostaglandins, thromboxanes, leukotrienes and prostacyclins, which are important biological regulators. These eicosenoids exhibit regulatory effects on lipoprotein metabolism, blood rheology, vascular tone, leukocyte function, platelet activation and cell growth. The application of arachidonic acid to an infant's diet is particularly important due to the rapid body growth of an infant. Arachidonic acid is an important precursor to many of the eicosanoids which regulate cellular metabolism and growth in infants. It is found naturally in human breast milk but not in most infant formula. In an effort to have infant formula match the long chain fatty acid profile found in breast milk, scientific and food regulatory bodies have recommended that arachidonic acid be added to infant formula, especially in formula utilized for premature infants.
In particular, it is preferable that arachidonic acid containing oil produced for use with infant formula contain little or no other long chain highly unsaturated fatty acids (e.g., eicosapentanoic acid). Such other long chain highly unsaturated fatty acids are not preferred because some of these fatty acids can interfere with the utilization of arachidonic acid by the infant, and/or can inhibit blending of the arachidonic acid-containing oil with other oils to achieve the appropriate ratio of fatty acids matching breast milk or other desired applications. Highly unsaturated fatty acids are defined as fatty acids containing 4 or more double bonds.
Traditional sources of arachidonic acid include poultry eggs, bovine brain tissue, pig adrenal gland, pig liver and sardines. The yield of arachidonic acid, however, is usually less than 0.2% on a dry weight basis. The use of microorganisms capable of producing arachidonic acid de novo have been suggested by various investigators, including Kyle, PCT Publication No. WO 92/13086, published Aug. 6, 1992; Shinmen et al., U.S. Pat. No. 5,204,250, issued Apr. 20, 1993; Shinmen et al., pp. 11-16, 1989, Appl. Microbiol. Biotechnol., vol. 31; Totani et al., pp. 1060-1062, 1987, LIPIDS, vol. 22; Shimizu et al., pp. 509-512, 1992, LIPIDS, vol. 27; Shimizu et al., pp. 342-347, 1989, JAOCS, vol. 66; Shimizu et al., pp. 1455-1459, 1988, JAOCS, vol. 65; Shimizu et al., pp. 254-258, 1991, JAOCS, vol. 68; Sajbidor et al., pp. 455-456, 1990, Biotechnology Letters, vol. 12; Bajpai et al., pp. 1255-1258, 1991, Appl. Environ. Microbiol., vol. 57; Bajpai, pp. 775-780, 1991, JAOCS, vol. 68; and Gandhi et al., pp. 1825-1830, 1991, J. Gen. Microbiol., vol. 137. The arachidonic acid productivity by the microorganisms disclosed by prior investigators, however, is less than 0.67 grams per liter per day. Such amounts are significantly less than the amounts of arachidonic acid produced by the microorganisms of the present invention. These lower productivity values are the result of employing strains: (1) with slow growth or lipid production rates leading to long fermentation times (i.e., greater than 2-3 days) (Kyle, 1992, ibid.; Shinmen et al., 1993, ibid.; Shinmen et al., 1989, ibid.; Bajpai et al., 1991, ibid.; Bajpai, ibid.; and Gandhi et al., ibid.); and/or (2) that contain low arachidonic acid contents (expressed as % fatty acids) in the final oil produced (Shinmen et al., 1993, ibid.; Shimizu et al., 1989, ibid.; and Kendrick and Ratledge, 1992, pp. 15-20, Lipids, vol. 27); and/or (3) which require long periods of stress (i.e., aging a biomass for 6-28 days) to achieve high levels of arachidonic acid in a biomass (Bajpai et al., 1991, ibid. and Shinmen et al., 1989, ibid.); and/or (4) that only exhibit high arachidonic acid content in non-commercial growth conditions (e.g., malt agar plates) (Totani and Oba, 1987, pp. 1060-1062, Lipids, vol. 22). In addition, non-Mortierella schmuckeri microorganisms that have been proposed for producing arachidonic acid, in particular Pythium insidiosum microorganisms, disclosed by prior investigators (Kyle, 1992, ibid.), have been reported to be pathogenic to humans and/or animals.
Thus, there remains a need for an economical, commercially feasible method for producing arachidonic acid. The present invention satisfies that need. There also remains a need for the an economical, commercially feasible food product for the introduction of arachidonic acid produced according to the present invention into the diet of human infants.